The present invention generally relates to a machine tool and more particularly, to a needle-roller type chuck for attaching a cutting tool or the like onto a main spindle of a machine.
Conventionally, for a needle-roller type chuck, there has been proposed, for example, in Japanese Utility Model Publication, Jitsukosho No. 41-23987, an arrangement as shown in FIGS. 1 and 2, which includes a chuck main body 1, a cylindrical portion 2 having a convergently tapered outer peripheral surface and extending forwardly (i.e. downwardly in FIG. 1) from the chuck main body 1, and a clamping nut 3 having an inner peripheral surface slightly larger in diameter than the outer peripheral surface of the cylindrical portion 2 and formed with a taper parallel to said outer peripheral surface, with said clamping nut 3 being mounted on the outer peripheral surface of said cylindrical portion 2 through a large number of needle rollers 5 rotatably supported in a retainer 4. In this arrangement, each of the needle rollers 5 is disposed in such a manner that its axis x is inclined so as not to intersect with the axis X of the chuck main body 1 for spiral revolution while rotating on the outer peripheral surface of the cylindrical portion 2 based on the rotation of the clamping nut 3, thereby to contract or restore the cylindrical portion 2 for selective attachment or detachment of a tool 6.
The known needle-roller chuck as described above, however, has various problems as follows.
(a) The cylindrical portion 2 which is subjected to a considerably large contracting force by the needle rollers 5 is required to have high rigidity and sufficient hardness, while it is also required to be provided with a large amount of deflection or flexibility in order to achieve an ample grasping capability. In other words, with respect to the cylindrical portion 2, there are involved such inconsistent problems that said cylindrical portion 2 must be thick to satisfy the former requirement, while simultaneously, it should be thin to meet the latter demand.
(b) It is necessary that the outer peripheral surface or needle roller contacting surface of the cylindrical portion 2 should have a hardness higher than HRC 60 from the viewpoint of durability. Meanwhile, at a shank portion 1a of the chuck main body 1, its surface hardness must be lower than that of the machine main spindle for the protection of said main spindle, and since such machine main spindle generally has a hardness at HRC 60, the shank portion 1a is required to have a hardness at approximately HRC 55 or thereabout. However, heat-treatment of the same item to have different hardnesses requires extremely complicated and troublesome steps, even though it is technically possible, thus not being suitable for practical applications.
Incidentally, the problems related to the above item (a) may be solved anyhow by forming a large number of slit-like axial grooves in the inner peripheral surface of the cylindrical portion 2 as disclosed, for example, in Japanese Utility Model Publications Jitsukosho Nos. 57-23055 and 58-15043, or by forming many axial deep holes in the cylindrical portion 2 from its forward end face as in Japanese Utility Model Publication Jitsukosho No. 57-33930. However, since both of these grooves and deep holes as described above are formed up to the forward end portion of the cylindrical portion 2 on which most of the stress is concentrated, upon grasping the tool 6 thereby, there are disadvantages in that not only the grasping force is lowered at such forward end portion, but also the so-called "collapsing deformation" is formed on the inner peripheral surface of the forward end portion (FIG. 1), thus resulting in an off-center vibration of the tool 6. Moreover, due to the fact that the grooves and deep holes as described above are open towards the inner peripheral surface or forward end portion of the cylindrical portion 2, chips, dust and dirt, cutting oils, etc. undesirably enter through such open portions, and removal thereof is extremely troublesome.
On the other hand, the problems relating to item (b) may also be solved by arranging the cylindrical portion 2 into a double wall construction, with its outer cylindrical portion contacting the needle rollers 5 being adapted to be detachable with respect to the chuck main body 1 as disclosed, for example, in Japanese Utility Model Publication Jitsukosho 59-33534. However, if the outer cylindrical portion is arranged to be detachable (e.g. fixing by set screws), there is a problem in that the deterioration in the setting or joint strength and the fixing accuracy between the outer cylindrical portion and the chuck main body 1 is inevitable.
In other words, up to the present, there have been proposed no needle-roller type chucks in which the problems in items (a) and (b) as stated so far, are solved without presenting any other new problems.